Big BA Bobby

I have a 2003 chevy 2500 hd, the instrument cluster definitely has a problem, but I have another problem, I can hook up a test to the negative side of the battery and touch the body with the light and it lights up, the light was dim but when I pulled the ipc/dic fuse it got really bright,anybody ever have a issue like this really need some help with this one, would be much appreciated thank you, I forgot to mention this truck already had this problem when I got it,previous owner took it to a repair shop and they could not find the problem so they put a big shut off switch on the negative cable so he could turn it off when he is not driving it, I accidentally found this problem with the test light when I was checking out the alternator, this happens with the switch off when you turn it on the light goes out, not sure what is going on here but some how getting positive feed to the ground, checked fuses and relays really got me puzzled please help thank you